Additive manufactured Ti6Al4V alloy has become a widely used metallic material in many structural applications including aerospace components and medical implants, where its balance of mechanical performance and flexibility in assuming complex shapes is very appreciated. As a consequence, extensive efforts have been directed towards its mechanical characterization and extended data collections are reported in the literature. However, in the basic characterization, some prominent aspects related to “true” stress-strain behaviour and strain rate effects appear to have been substantially neglected. In this work the tensile behaviour of a Ti6Al4V alloy produced by Electron Beam Melting is investigated at static and dynamic rates together with the torsion response at static rates, in order to evaluate the effects of the strain rate and the Lode stress-state parameter on the elastoplastic response and the fracture initiation. The experimental procedure adopted here relied on enhanced true stress-true strain measurements based on optical measurements of the necking cross section of the specimen, continuously acquired by video recordings, as an alternative to the traditional elongation-based approach to the true stress-true strain evaluation. In the case of static torsion tests, the Nádai method has been used, delivering the equivalent stress-strain curves directly from the torque-rotation experimental data. Moreover, the real stress amplification due to high strain rates has been obtained subtracting the thermal softening effect from the flow curves of the dynamic tests. In addition to the mechanical characterization, micrographic analyses allowed to relate the microstructural properties to the additive process conditions and the fracture morphology to the various test conditions imposed to different specimens. Finally the stress-strain results from experiments are compared with literature data for Ti6Al4V alloy, manufactured by the same or different production processes, evidencing how not only manufacturing technique and process conditions, but also the approach used in the characterization affects the results of material behaviour assessment.

Analysis and modelling of tensile and torsional behaviour at different strain rates of Ti6Al4V alloy additive manufactured by electron beam melting (EBM)

Mirone G.
;
Barbagallo R.;Giudice F.;
2020-01-01

Abstract

Additive manufactured Ti6Al4V alloy has become a widely used metallic material in many structural applications including aerospace components and medical implants, where its balance of mechanical performance and flexibility in assuming complex shapes is very appreciated. As a consequence, extensive efforts have been directed towards its mechanical characterization and extended data collections are reported in the literature. However, in the basic characterization, some prominent aspects related to “true” stress-strain behaviour and strain rate effects appear to have been substantially neglected. In this work the tensile behaviour of a Ti6Al4V alloy produced by Electron Beam Melting is investigated at static and dynamic rates together with the torsion response at static rates, in order to evaluate the effects of the strain rate and the Lode stress-state parameter on the elastoplastic response and the fracture initiation. The experimental procedure adopted here relied on enhanced true stress-true strain measurements based on optical measurements of the necking cross section of the specimen, continuously acquired by video recordings, as an alternative to the traditional elongation-based approach to the true stress-true strain evaluation. In the case of static torsion tests, the Nádai method has been used, delivering the equivalent stress-strain curves directly from the torque-rotation experimental data. Moreover, the real stress amplification due to high strain rates has been obtained subtracting the thermal softening effect from the flow curves of the dynamic tests. In addition to the mechanical characterization, micrographic analyses allowed to relate the microstructural properties to the additive process conditions and the fracture morphology to the various test conditions imposed to different specimens. Finally the stress-strain results from experiments are compared with literature data for Ti6Al4V alloy, manufactured by the same or different production processes, evidencing how not only manufacturing technique and process conditions, but also the approach used in the characterization affects the results of material behaviour assessment.
2020
High strain rate
Lode angle
Nádai method
Split hopkinson tensile bar (SHTB)
Titanium alloy
True curve
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.11769/487365
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